I presume (hope) that Windows 10 will deliver a new interface for a Windows Phone device when connected via USB or Bluetooth to a Windows desktop / 2in1 / tablet device.
I hope so because the existing interface between WP8.1 and Windows x is quite frankly appalling and embarrassing on its own but especially when compared with the current version of iTunes and the integration between current versions of OSX and iOS.
I have a concept for this interface (which may already be in consideration / engineering; if it is, excellent) which is unique in the phone / mobile space and would create a true differentiator.
Best of all the technology and a lot of the code exists and has already been previewed.

When a users Win Phone device connects a Win desktop (or tablet, 2in1 etc.) device via USB or is paired and comes within range of Bluetooth, a notification pops up on the desktop device asking the user if they want to connect their device. There could be optional security implemented here such as having to enter the phones pin code either on screen on or the device etc.
If the user agrees the phones screen is mirrored in a window on the PC. This has already been previewed with the very limited functionality 'project my screen' project but I believe this should become the default interface for Windows 'Phone' devices. Unlike the previous implementation, full PC keyboard and mouse functionality would be implemented which would allow the user to use any phone app on his PC screen. Phone audio should be carried in and out of the desktop, the desktop FF camera if available should be available to the phone apps etc.

This is very a very different and unique approach to other ecosystems, Apple have made big advances in integrating OSX and iOS and Windows must have a strong showing to compete. The reason this is the perfect interface app is that there is no interface, the UI for the desktop app is the phones UI and the user is automatically comfortable with that. There is zero learning curve.

There would be extra functionality built into the interface, for instance, the user could drag and drop files, music, etc. to and from the phone via the interface and extended functionality, for example the user could open the phones music app on the desktop display, right click on a track which is physically located on the phone, rename it or have full metadata editing of the track etc.
It would also be amazing if the window the phone was being viewed in could be expanded, e.g. the user opens an excel spreadsheet on the phone interface and can then grab the corner of the 'phone' window and drag the window wider / taller which will allow the user to view more of the spreadsheet or drag the window wider to see the other pivots of an app all at the same time. Technically I dont know if this is possible but it would make a **** of a reveal when demoing the interface software, especially the Excel / Word implementations..

The interface app should be freely available and also be installable on the desktop etc. without administrator requirements which is very important. It should be available and have functional commonality wherever possible across Windows 7,8 and 10.
I believe people would love to be able to use whatsapp and other pure mobile apps on their desktop without even taking their phone out of their bag whilst maintaining their conversation history etc. in their phone, this would provide exactly that functionality.